Beam homogenizer

ABSTRACT

A beam homogenizer that minimizes undesired intensity variations at the output plane caused by sharp breaks between facets in previous embodiments. The homogenizer includes a hologram made up of irregularly patterned diffractive fringes. An input beam illuminates at least part of the hologram. The hologram transmits a portion of the input beam onto an output plane. In doing so, the energy of the input beam is spatially redistributed at the output plane into a homogenized output beam having a preselected spatial energy distribution at the output plane. Thus, the illuminated portion of the output plane has a shape predetermined by the designer of the homogenizer.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation application of pending application Ser. No.09/160,322 filed Sep. 25, 1998, now U.S. Pat. No. 6,025,938, which is acontinuation of Ser. No. 08/770,524, filed Dec. 20, 1996 now U.S. Pat.No. 5,850,300, which is continuation-in-part application of Ser. No.08/203,188 filed Feb. 28, 1994 is now U.S. Pat. No. 5,610,733.

FIELD OF THE INVENTION

The invention relates generally to an optical apparatus, and pertainsmore specifically to a system for producing an output beam having apreselected distribution of power and/or energy while minimizingundesired intensity variations at the output plane caused by sharpbreaks between facets.

BACKGROUND OF THE INVENTION

A laser device generally produces a beam of coherent light that has awavefront of relatively small cross-section. In spite of the smallcross-section and the coherency of the beam, the wavefront of a lasertypically has a nonuniform power distribution that is stronger in thecenter than at the outer edges. The power variation may be between fiveand ten percent Furthermore, to make use of the beam, it is oftennecessary to expand the cross-sectional area of the beam, therebyspreading the non-uniformity across a larger wavefront.

When conventional lenses are used to expand the beam, the non-uniformpower distribution of the wavefront is carried through to the expandedbeam. In addition, the non-uniformity of the beam becomes more apparentas the wavefront is now expanded over a greater cross-sectional area.This non-uniformity is often detrimental to the performance of a systemutilizing the beam as the system must be designed for some average levelof beam power or another approach would be to somehow strip the beam ofits lesser power outer portions, possibly through the use of anaperture. Neither of these alternatives enable optimum use of the beam'spower and it is very difficult to achieve a uniform power distribution,such as the plus or minus one percent variation that is often desired,by way of conventional lens systems.

Holographic elements have been created to function as conventional bulkoptical elements. In these cases, the holographic element, whoseorientations and spatial periods are correct for the purpose ofdiffracting the incident wavefront into a desired output locationpattern, shape or image. However, when built to function as a basiclens, these holographic elements would also carry the nonuniform powerdistribution through to the output pattern, shape or image, thereby alsoinefficiently using the power of the optical source.

The problem of how to compensate for wavefronts having a nonuniformpower distribution is addressed U.S. Pat. No. 4,547,037. In this patentdiscloses a multi-faceted holographic element which redistributes thelight energy of an incident beam onto a second plane disclosed. This isaccomplished by constructing each facet as an individual hologram ordiffraction grating. Each facet is sized to be inversely proportional tothe intensity of the portion of the beam incident thereupon to assuringthat substantially the same amount of power passes through each facet.The light transmitted through each facet is diffracted to arrive atdifferent locations on a second plane, relative to their locations inthe holographic element. Each of the subholograms or diffractiongratings either expand or contract the portion of the incident beampassing therethrough to illuminate equal, but different, areas on thesecond plane, thereby producing an output beam at the second plane witha wavefront of nearly constant intensity.

A problem with devices incorporating the teachings of the '037 patent isthat if the power distribution of the incident beam upon the surface ofthe hologram deviates from the design parameters, then the powerdistribution of the output beam at the second plane will be similarlyaffected and thus no longer uniform. In optical systems, there are manycauses for such deviation in the power distribution of the incident beamcould occur. For example, power fluctuations due to the age of thecomponents, or simply the replacement of the source due to failure. Inaddition, any misalignment within the system due to shock or age willproduce an output wavefront having a non-uniform power distribution.

What is needed is an relatively inexpensive way to convert an incidentoptical beam having a wavefront with a non-uniform spatial energydistribution to an output beam having a substantially uniform spatialenergy distribution that is relatively insensitive to fluctuations inpositioning of the incident beam and spatial energy distributions withinthe incident beam.

Further, what is needed is a relatively inexpensive way to convert anincident optical beam having a wavefront with a non-uniform spatialenergy distribution to an output beam having a preselected spatialenergy distribution using a hologram that does not have regular breaksbetween facets in order to better minimize the intensity variations onthe output plane caused by regular breaks between facets.

Further, what is needed is a relatively inexpensive way to convert anincident optical beam having an arbitrary wavefront to an output beamhaving preselected attributes, including preselected angular spread,such that the output beam is useful in photolithography.Photolithographic exposure systems are used to image the pattern of amask onto a wafer for the purposes of exposing resist, or photoresist,on the wafer in a pre-determined pattern. Subsequent processing of thewafer results in the completion of layers that eventually form thedesired device, such as an integrated circuit.

When the mask is used in a projection lithography system, such as alaser stepper with a 5:1 or 10:1 reduction ratio, the mask is oftenreferred to as a reticle. The reticle or mask is typically formed bychrome regions on a transparent substrate. The chrome regions of themask block the incident light, thereby imposing the pattern of the maskas an intensity variation on the light. In a 5× laser stepper, thepattern of the reticle is reduced by a factor of 5 as imaged onto awafer. Typically, in this application, the beam illuminating thediffractive is relatively uniform and has a rather narrow cone angle ofdivergence, i.e., limited spatial and angular energy distributions.

While masks and reticles control the intensity of light on the wafer,there is a need for an element that controls the angular distribution ofthe light on the wafer. By modifying the particular angular distributionof the light illuminating the wafer, one can extend the depth of thefield and resolving power of photolithographic exposure systems. Thiselement should ideally be inexpensive and relatively insensitive tofluctuations in positioning of the incident beam and to fluctuations inthe spatial energy distributions of the incident beam.

Moreover, what is needed is a relatively inexpensive way to convert acollimated incident optical beam having a wavefront with non-uniformspatial energy distribution to an output beam having a preselectedspatial energy distribution, or a preselected beam shape, that isrelatively insensitive to fluctuations in positioning of the incidentbeam and spatial energy distributions within the incident beam.Additionally, what is needed is a relatively inexpensive way to convertan incident optical beam having a wavefront with non-uniform spatialenergy distribution to an output beam having preselected attributes,such as spatial energy distribution, or a preselected beam shape, or apreselected angular energy distribution, that is relatively insensitiveto fluctuations in positioning of the incident beam and spatial energydistributions within the incident beam.

SUMMARY OF THE INVENTION

The invention is a beam homogenizer for converting an incident beamhaving a non-uniform spatial energy distribution into an output beam ofpreselected spatial energy distribution. The incident beam is incidentupon the beam homogenizer, formed as an array of facets where each facetis constructed to transmit any portion of the incident beam incidentthereupon to an output plane spaced from the beam homogenizer so thatthe light transmitted through each of the facets overlap at the outputplane to form the output beam which now has a substantially uniformspatial energy distribution.

Additionally, the invention is a beam homogenizer that minimizesundesired intensity variations at the output plane caused by sharpbreaks between facets. At least part of a hologram comprisingirregularly patterned diffractive fringes is illuminated by an inputbeam. That part transmits a portion of that beam onto an output plane,whereby the energy of the input beam is spatially redistributed at theoutput plane into a homogenized output beam having a preselected spatialenergy distribution at the output plane such that the illuminatedportion of the output plane is a predetermined shape and a predeterminedmagnitude.

Moreover, the invention is a beam homogenizer for converting a inputbeam having a non-uniform spatial energy distribution into an outputbeam having a preselected spatial energy distribution at an output planewhile minimizing the intensity variation caused by breaks betweenfacets. An input beam illuminates at least some of the facet areas of ahologram. The facet areas have irregularly patterned diffractivefringes. The facet areas transmit a beam such that at an output plane,the majority of the portion of the input beam transmitted through eachof said illuminated facet areas overlaps the portion of the input beamtransmitted through at least one other illuminated facet whereby theenergy of the input beam is spatially redistributed at the output planeinto a homogenized output beam having a preselected spatial energydistribution at the output plane. The array of facet areas is acomputer-generated hologram, relatively insensitive to fluctuations inpositioning of the input beam for incidence thereupon and to spatialenergy distributions within the input beam. The homogenizer transmitsthe transmitted portion of the input beam at a preselected angularspread and illuminates a target area corresponding to a preselectedspatial energy distribution desired at the output plane.

Additionally, the invention is a beam homogenizer system for convertingan input beam having a non-uniform spatial energy distribution into anoutput beam having a preselected spatial energy distribution at anoutput plane while minimizing the intensity variation caused by breaksbetween sub-holograms. An input beam illuminates at least some of anarray of computer generated subholograms whose size is determinedindependently of the intensity of the portion of the input beam incidentthereupon, and being relatively insensitive to fluctuations inpositioning of the input beam for incidence thereupon. Each sub-hologramdiffracts a majority of the portion of the input beam incident thereuponso that at a target located at the second plane, the portion of theinput beam diffracted by each of the illuminated sub-holograms overlapsthe portion diffracted by at least one other illuminated computergenerated sub-hologram to form an output beam. The intensity of theoutput beam is substantially equalized over a entire target. The outputbeam has a preselected angular spread and the target corresponds to apreselected spatial energy distribution desired at the output plane.

Additionally, the invention is a method of homogenizing an input beamhaving an arbitrary spatial energy distribution at a first plane into anoutput beam with a preselected spatial energy distribution at a secondplane while minimizing the intensity variation caused by breaks betweensub-holograms. Steps taken are providing a holographic optical elementcomprising an array of computer generated sub-holograms with irregularlypatterned diffractive fringes, fixedly positioning the element at thefirst plane so that the input beam illuminates at least some of thesub-holograms, each illuminated sub-hologram expansively diffracting theportion of the input beam incident thereupon over an entire target atthe second plane to superimpose the diffracted portions of all of theilluminated sub-holograms to form an output beam at the second plane,wherein the step of providing the holographic element comprisesgenerating an array of sub-holograms that is relatively insensitive tofluctuations in positioning of an input beam for incidence on said arrayand to spatial energy distributions within the incident beam. In theinvention, each illuminated sub-hologram expansively diffracts theportion of the input beam incident thereupon at a preselected angularspread and produces a preselected spatial energy distribution desired atthe output plane.

Additionally, the invention comprises a beam homogenizer system forconverting an incident beam having an arbitrary spatial energydistribution into an output beam having preselected spatial energydistribution at an output plane spaced from the homogenizer whileminimizing the intensity variation caused by breaks betweensub-holograms. An array of sub-holograms designed with an iterativeencoding method such that each sub-hologram has irregularly shapeddiffractive fringes, and such that portions of incident beam diffractedby several of said sub-holograms overlap at the output plane, wherebythe output beam has a preselected spatial energy distribution that isrelatively insensitive to fluctuations in positioning of an input beamfor incidence on the homogenizer and to spatial energy distributionswithin the incident beam. Each sub-hologram transmits a beam with apreselected angular spread. The output beam has a preselected spatialenergy distribution desired at the output plane.

Additionally, the invention is a beam homogenizer system for convertingan incident beam having an arbitrary spatial energy distribution andlimited angular energy distribution into an output beam having apreselected angular energy distribution while minimizing the intensityvariation caused by breaks between sub-holograms. An array ofsub-holograms, each of said sub-holograms having irregularly shapeddiffractive fringes, and each of said sub-holograms containing pixelsthat exhibit phase skipping and the light diffracted by at least two ofthe sub-holograms overlap in the output plane to form an output beam.The output beam has a preselected angular spatial energy distributionthat is relatively insensitive to fluctuations in positioning of aninput beam for incidence on said homogenizer and spatial energydistributions within the incident beam. The output beam has apreselected spatial energy distribution and/or a preselected angularenergy distribution.

Additionally, the invention is a beam homogenizer for converting aninput beam of non-uniform spatial distribution into an output beam of amore-uniform distribution. A computer-generated hologram in theinvention has a phase-transmittance pattern. The Fourier Transform ofthe phase-transmittance pattern is uniform over a preselected angularregion. The pattern is made up of one or more binary phase elements.

Additionally, the invention is a system for modifying the angular spreadof an incoherent or partially coherent beam of light. An incident beampropagating with a cone angle is diffracted by a diffractive diffusingelement into a range of preselected angles. These angles are determinedby or dictated by the cone angle of the incident beam and the FourierTransform of the diffusing element.

Additionally, the invention is a photolithographic-optical system. Aninput beam illuminates a diffractive diffusing element. The diffractivediffusing element illuminates a mask by the element's transmission of anoutput beam at a preselected angular distribution.

It is an object of this invention to convert an incident optical beamhaving a non-uniform spatial energy distribution to an output beamhaving uniform spatial energy distribution at an output plane.

It is a further object of this invention to convert an incident beamhaving a non-uniform spatial energy distribution into an output beamhaving a preselected spatial energy distribution at an output planespaced from the homogenizer while minimizing the intensity variationcaused by breaks between facets.

It is a further object of this invention to convert an incident beamhaving a non-uniform spatial energy distribution into an output beamhaving a preselected spatial energy distribution of a preselected shapeat an output plane spaced from the homogenizer.

It is a feature of this invention that the optical beam having anon-uniform spatial energy distribution incident upon a homogenizerhaving an array of facets and the portion of the incident beamtransmitted through each facet is imaged over an entire target onoverlap at an output plane, thereby homogenizing the incident opticalbeam to produce an output beam of substantially uniform powerdistribution at the output plane. It is another feature of thisinvention that the homogenizer is a hologram and each of the facets aresubholograms. It is yet another feature of this invention that thesubholograms are designed to minimize interference effects at the outputplane between the light transmitted through the facets.

It is a feature of this invention that the incident beam having anon-uniform spatial energy distribution is converted into an output beamhaving a preselected spatial energy distribution at an output planespaced from the homogenizer while minimizing the intensity variationcaused by breaks between facets. It is a further feature of thisinvention that an incident beam having a non-uniform spatial energydistribution is converted into an output beam having a preselectedspatial energy distribution of a preselected shape at an output planespaced from the homogenizer.

It is an advantage of this invention that the homogenizer may bedeveloped by computer generation techniques and may be fabricatedrelatively inexpensively. It is another advantage of this invention thatthe homogenization is relatively insensitive to fluctuations in thepower density of the incident beam. It is a further advantage of thisinvention that the intensity of the output beam is substantiallyinsensitive to the location the incident beam falls on the homogenizer.

It is a further advantage of this invention that it can convert anincident beam having a non-uniform spatial energy distribution into anoutput beam having a preselected spatial energy distribution at anoutput plane spaced from the homogenizer while minimizing the intensityvariation caused by breaks between facets. It is a still furtheradvantage of this invention that the invention can convert an incidentbeam having a non-uniform spatial energy distribution into an outputbeam having a preselected spatial energy distribution of an arbitrarypreselected shape at an output plane spaced from the homogenizer.

It is a still further advantage of this invention that it can convert anincident beam having arbitrary spatial energy distribution and limitedangular energy distribution into an output beam of preselected angularenergy distribution or of preselected shape at an output plane spacedfrom the homogenizer.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described by way of example withreference to the accompanying drawings in which:

FIG. 1 shows the spatial energy distribution of a beam incident upon ahomogenizer of the present invention and how the portions of theincident beam that are transmitted through the homogenizer areconstituted at an output plane to produce an output beam having asubstantially uniform power distribution.

FIG. 2 shows the spatial energy distribution of an incident beam that istypical of a excimer laser incident on a homogenizer and the resultingoutput beam.

FIG. 3 shows yet another spatial energy distribution of an incident beamthat is typical of a Nd:YAG laser incident on a homogenizer and theresultant output beam.

FIG. 4 shows the spatial energy distribution of a input beam incidentupon a homogenizer of the present invention, how the homogenizer hasirregularly patterned plateaus and vias, and how the portions of theincident beam that are transmitted through the homogenizer areconstituted at an output plane to produce and output beam having asubstantially stable power distribution and having a preselected powerdistribution shape of a circle.

FIG. 5A shows a hologram having multiple facets and showing the sharpbreaks between facets.

FIG. 5B shows a close-up view of the hologram shown in FIG. 5A such thatthe sharp breaks between facets are seen in greater detail.

FIG. 6 shows the transmission of a beam with a preselected angularspread of 20 degrees from a facet area to the output plane.

FIG. 7 shows a magnified view of the hologram shown in FIG. 4, showingthe irregularly patterned plateaus and vias, and showing the 4×4 arrayof facet areas of arbitrarily shown size, and showing that the facetsare not repeated patterns and exhibit no discontinuities at facetboundaries.

FIG. 8 shows a circular target pattern, that is, a preselected powerdistribution pattern in which the spatial power is distributed at theoutput plane in a circle with relatively no distribution around thatcircle.

FIG. 9 shows a doughnut-shaped target pattern at the output plane, thatis, a preselected power distribution pattern in which the spatial poweris distributed at the output plane such that a relativelynon-illuminated circular area is surrounded by a ring of illuminatedarea, which is in turn adjacent to a relatively non-illuminated area.

FIG. 10 is a side-view showing the present invention in use in aphotolithography system.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention is shown in FIG. 1. An optical beam is incident ona homogenizer 10 having an array 11 of facets 12. Each of these facets12 are constructed to direct any portion of an incident optical beam 14that is incident thereupon uniformly over an entire target 16 at anoutput plane of 18. As the portion of the incident beam transmittedthrough each of the facets 12 (shown illustratively as 20 a and 20 b)overlap at the target 16, the incident optical beam is mixed, therebyhomogenizing the discrete portions of the incident beam 14 that are thetransmitted through each facet 12. This homogenization assures that atthe target 16 there is a uniform mix of the incident beam 14, such thatat the output plane 18 the output beam 22 has a uniform powerdistribution 24. The homogenization process averages the power of theincident beam 14 with some losses due to inefficiencies.

The incident optical beam 14 emanates from an optical source (notshown), such as a laser, and is preferably in a collimated state, thespatial energy distribution of the incident beam may take on variousforms, some of which are illustrated in FIGS. 1-3. The incident opticalbeam 14 may be transferred to the homogenizer 10 from the optical sourceand collimated by way of conventional bulk optical elements, such aslenses and mirrors, or through the use of holographic elements thatproduce the same results as conventional optical elements. Typically,the incident beam 14 has a power distribution 26 that may have asignificant intensity variation across its cross-section. In somesources the intensity distribution 26 variation could be between fivepercent and ten percent. In addition, as the optical source ages or thepower supply driving the optical source fluctuates, the powerdistribution 26 of the incident beam 14 may also change. The beamhomogenizer 10 is constructed to blend the incident optical beam 14 sothat at the target 16 of the output plane 18 the output beam 22 willhave an essentially uniform power distribution 24, independent of anyvariation in the power distribution 26 of the incident beam 14 andregardless of slight variation in where the incident beam 14 falls onthe array 11. The power distribution 24 at the target 16 will beessentially an average of the power uniformity over each facet 12 ratherthan the power uniformity 26 of the incident signal 14.

The beam homogenizer 10 in this embodiment is a holographic element andthe facets 12 are sub-holograms, shown in the FIGURE as a M×N lineararray of equal sized sub-holograms. It is also envisioned that thesub-holograms may be of different sizes. These sub-holograms 12 areconstructed to diffract any portion of the incident optical beam 14 thatis incident thereupon over the entire target 16 at the output plane 18.Each of these sub-holograms 12 is a distinct diffractive grating thatwill direct the portion of the incident beam 14 over the entire target16. Due to the number of fringes or pixels, which would approach eightylines per millimeter (80 lines/mm) for the sub-holograms 12 in theparticular implementation described below, these fringes have beenomitted from the drawing for the sake of clarity. In addition, thehomogenizer 10 is shown as a four-by-four array 11 for clarity ofillustration and ease of description, while in reality, a describedbelow, there may be substantially more sub-holograms 12, or facets,making up the homogenizer 10.

One way of fabricating holograms is by creating an interference patternamong coherent light beams on a photographic plate and then developingthe plate. Interference based holograms contain internal features whichproduce the interference fringes, making mass production at this timedifficult.

In order to overcome the production problems associated withinterference based holograms, Computer Generated Holograms (CGH) havebeen developed. CGH's may be developed by calculating the desiredholographic pattern and then, based upon the given constructionconditions, mathematically working backwards from that pattern, orreconstructed wavefront, to the particular hologram required. Severaliterative CGH encoding methods have been developed to take advantage ofthe increased performance of computers to develop CGHs withsignificantly higher performance than holograms developed using othermathematical techniques.

CGHs are usually surface-relief in nature and CGHs are formed usingphotolithographic, etching, electron-beam writing or other techniques.The electron-beam technology provides resolution close to that ofoptical film, but contains amplitude and phase quantization levels thatare much coarser. Photolithic procedures can provide multilevelholograms; however, alignment error between the layers increases withthe number of layers.

Note that the major difference between the present invention and that ofU.S. Pat. No. 4,547,037 is that in the latter the light illuminatingeach facet is directed to a different location in the output plane. Onthe other hand, in the present invention light from many facets willoverlap in the output plane. While the advantages of this feature wereexplained earlier, the disadvantage of this feature is that in locationsin the output plane where light from several facets overlap, coherenceeffects can cause interference patterns to arise that could cause largefluctuations in the laser intensity profile, if the holograms are notspecifically designed to avoid this problem. For example, if thesubhologram were designed independently, and the size of eachsubhologram was made smaller than the spatial coherence width of thelaser source, then the coherence effects could cause large bright anddark fringe patterns in locations where the light from several facetsoverlap.

This problem can be reduced by designing the subholograms with aniterative encoding method such as Iterative Discrete On-axis (IDO)encoding. This method is more fully described in the publicationentitled Iterative Encoding of High-Efficiency Holograms for Generationof Spot Arrays, Optics Letters, Vol. 14, pp. 479-81, 1989 by co-inventorFeldman et al. the disclosure of which is hereby incorporated byreference. Briefly, the hologram is divided into a two-dimensional arrayof rectangular cells and transmittance values for each cell is chosenand then optimized until an acceptable image is obtained. During theoptimization process, the image, including interference effects betweendifferent facets, are monitored. The transmittance values for each cellis chosen to not only spread the light illuminating each facet over alarge portion of (or the entire) output plane, but also to minimize theinterference effects among the facets. Since on-axis encoding does notrequire a carrier wavefront for the hologram to function, theseholograms can produce CGH's with much higher diffraction efficienciesthan off-axis methods which do require a carrier wavefront. This isbecause holograms have a practical upper limit to the availableSpace-Bandwidth Product (SBP), or information contained in the CGH, thatcan be used to encode the desired image. When no information is requiredfor a carrier wavefront, more information may be encoded relative to thedesired image. It may also be desirable to use the encoding methoddescribed in U.S. Pat. No. 5,202,775 titled Radially Symmetric Hologramand Method of Fabricating the Same which is also incorporated herein byreference. One usual characteristic of iterative encoding methods suchas IDO and RSIDO is that of “phase skipping.” Phase skipping, describedalso in U.S. Pat. No. 5,202,775 occurs when two adjacent CGH pixels havephase levels that differ by more than one phase level but less than byN−1 phase levels. Note that phase skipping does not occur when binary ormulti-level gratings are employed such as those described in U.S. Pat.No. 4,547,037.

In applying the IDO method to the design of each sub-hologram 12, it isimportant to keep the diffraction angles small so that a high efficiencyCGH, with physically realizable features, can be developed based on theshort wavelength of the incident optical beam 14. For this particularexample, the incident optical beam 14 is assumed to have an ellipticalform of 2.5 cm by 1 cm (centimeter) with a wavelength of 308 nm±1 nm(nanometer). The target 16, or output beam 22, could take on a number ofshapes including circular or square and in particular in this describedexample the diameter at the output plane 18 is selected to be 1.5 cm.Under these conditions the maximum deflection angle of the beamhomogenizer 10 will be 2.9° if the spacing between homogenizer 10 andthe target 16 on the output plane 18 is 20 cm.

With the maximum CGH deflection angle being 2.9°, a maximum spatialfrequency of 160 lp/mm (line-pairs/millimeter) is required. In order tohave an economical use of the optical power of the source, in this casea laser (not shown), the CGHs that make up the homogenizer must havehigh diffraction efficiency. A diffraction efficiency of approximately80% to 90% would be obtainable if the CGH spatial frequency is 800lp/mm, or approximately four times the maximum spatial frequencyrequired. The CGH spatial frequency of 800 lp/mm corresponds to a CGHminimum feature size of 0.6 μm (micrometer).

A further requirement to enable the CGH to have the high efficiencyneeded for economical use of the power of the incident beam is that theSBP (Space Bandwidth Product) of each sub-hologram be greater than orequal to 128×128. SBP is the number of pixels in the subhologram. It isalso a measure of degrees of freedom. In general, a large number ofdegrees of freedom are needed to implement arbitrary optical functionswith high efficiencies. This places a lower boundary on the dimensionsof each of the subholograms of 77 μm×77 μm. With the dimensions of eachsubhologram set to 100 μm×100 μmA a 100×100 facet array is of sufficientsize to be used with the beam of the present example. These particularparameters yield a SBP of 167×167 well above the projected minimum SBPof 128×128 required for a diffraction efficiency between 80-90%. Thecalculated final diffraction efficiency for this device is projected tobe between 85% and 95%.

The transmittance 20 a, 20 b of each sub-hologram 12 will cover theentire target 16 at the output plane 18 and form the homogenized outputbeam 22. It is anticipated that the output beam 22 diameter will be 1.5cm×1.5 cm. In this case the output beam 22 is of a smaller diameter thanthe input beam 14. It would also be possible for the output beam 22 tobe expanded by the homogenizer 10 such that the target 16 will have alarger cross-section than the input beam 14 or any arbitrary profiledesired.

The output plane 18 represents an area in space rather than anyparticular element. It would be possible to place a bulk opticalelement, an optical fiber, another hologram, an active device or anyother apparatus that would make use of the output beam, such as ablocking mask or an object to be illuminated. One such application wouldbe to incorporate an optical element at the output plane 18 that wouldenable the output beam to be used in laser cutting machines. In theabsence of the homogenizer 10, a beam used in laser cutting applicationshas the intensity distribution of the incident beam 24 or a significantamount of the power of the incident beam will be lost by passing thebeam through an aperture. As shown in FIG. 1, the wavefront of theincident beam has a higher power center section, or “hotspot”, that willcut through material faster than the lesser power outer fringe section.This makes for less accurately cut edges since the edge would take on ashape that approximates the reciprocal of the wavefront of the incidentbeam power distribution. The power distribution of the output wavefrontat the target illustrates the crisp power difference between off-targetand on-target intensities of the homogenized beam. With the homogenizedpower distribution of the homogenized beam, cutting occurs moreuniformly across the output beam to produce a more accurate edge.

Another embodiment of the present invention is shown in FIG. 4. Thisembodiment, like the embodiments described previously, homogenizes thespatial power distribution at the output plane. This embodiment, though,eliminates the sharp edges or breaks between facets of the prior art andthe embodiments described above, thusly eliminating intensity variationat the output plane caused by such sharp edges or breaks. Thisembodiment also transmits the beam incident upon the homogenizer at apreselected angular spread or angular divergence. Also, the embodimentshown in FIG. 4 illuminates preselected, shaped target areas at theoutput plane. Another way to describe or designate the preselected,shaped target area is that it is a preselected spatial powerdistribution at the output plane. Still another way to describe it isthat it is a shaped pattern beam at the output plane. All of the designand manufacturing methods and features described for use in previousembodiments are equally applicable to the embodiment shown in FIG. 4.

Sharp edges or breaks 48 between facets 12, as seen in FIG. 1, FIG. 2,and FIG. 3, are structure on the homogenizer. FIG. 5A shows a hologram46 in which sharp edges or breaks 48 between facets 12 can be clearlyseen. FIG. 5B shows a close-up view of the sharp edges or breaks 48between the facets 12 of the hologram 46 shown in FIG. 5A. Noteadditionally that interaction between diffractive fringes, made up ofplateaus 50 (shown as white areas) and vias or valleys 52 (shown asblack areas) in the preferred embodiment, at the edges of the facetscreate some undesired structure at the output plane.

Such structure 48 on the homogenizer causes some undesired diffractionof the input beam incident upon the homogenizer. Regularly patterned, orregularly repeated, such structure diffracts the incident beam 14 suchthat undesirable distinct, repeated intensity variation in spatial powerdistribution appears at the output beam at the output plane.

The embodiment shown in FIG. 4 removes such sharp edges or breaks andthusly minimizes the intensity variations caused by such edges orbreaks. The embodiment shown in FIG. 4 replaces the regularly patternedfacet array 11 of previous embodiments with a optical instrument 54,such as a hologram, having irregularly patterned diffractive fringes 70or diffractive gratings. The diffractive fringes 70 of the preferredembodiment are made up of plateaus 50 (shown as white areas) and vias orvalleys 52 (shown as black areas), and the diffractive fringes will bereferred to herein by reference to plateaus and vias. One of ordinaryskill in the art of this invention will recognize the construction ofholograms of plateaus and vias that approximate lenses. U.S. Pat. No.4,895,790 discloses the construction of optical elements having plateausand vias, and the disclosure of that patent is incorporated herein.Also, U.S. Pat. No. 5,202,775 discloses a method of fabricatingholograms and the disclosure in that patent is incorporated herein.

A homogenizer comprising a hologram having irregularly patternedplateaus 50 and vias 52 no longer has regular sharp edges or breaks tocause undesired regular and repeated intensity variation on the outputplane. This is shown in FIG. 4.

Facet area, for purposes of the invention shown in FIG. 4, refers to anarea of arbitrarily designated size on the hologram 54. It is used as aconvenient way to refer to an area of irregularly shaped or patterneddiffractive fringes (irregularly shaped or patterned plateaus and viasin the preferred embodiment) on the hologram. A hologram has at leasttwo facet areas. In the preferred embodiment, no two facet areas 56contain a pattern of plateaus 50 and vias 52 that are alike. Anotherconvenient way to refer to an area of irregularly shaped plateaus andvias on the hologram is to refer to the area as a facet. The patternwithin one facet is nominally correlated to itself and nominallyuncorrelated to the pattern in all other facets. Therefore, each facetwithin the hologram directs light to the entire target area of theoutput plane.

An enlarged, frontal view of the hologram 54 of FIG. 4 is shown in FIG.7. The hologram 54 of FIG. 7 (and the hologram 54 of FIG. 4) have beenarbitrarily designated to have an array of facet areas, or facets, 56 of4×4 as shown by dotted lines in FIG. 7. These dotted lines are notstructure on the hologram, but are used to designate a facet area or afacet of this embodiment, which is a convenient way to refer to an areaof the hologram 54 of this embodiment. Each facet area 56 hasirregularly patterned plateaus 50 and vias 52. No sharp edges or breaksappear between facet areas 56. The dotted lines are shown designatingonly one of the sixteen facet areas 56 in FIG. 4 because the dottedlines designating the other fifteen would be hard to see and wouldconfuse if shown in FIG. 4.

Referring to FIG. 4, when the incident optical beam, preferably acollimated beam, illuminates the facet areas, or facets, 56 of thehologram 54, the irregularly patterned plateaus 50 and vias 52 provideno regular, undesired structure to transmit regular, undesired intensityvariation (not shown) in the output plane 18. An additional advantage ofthe present embodiment is that departing from designing each individualfacet 56 to designing facet areas or entire holograms provides greaterfreedom of design that allows the designer to reduce undesired intensityvariation in the output plane by making adjustments to the plateaus 50and vias 52, whereas the sharp breaks or edges of the discrete facetembodiment provided much less design freedom in relation to those sharpbreaks or edges. An additional advantage of the present embodiment isthat departing from designing each individual facet 56 to designingfacet areas or entire holograms provides greater freedom of design thatallows greater freedom to create output beams of arbitrary shapes, suchas rings and cross-hairs.

In the embodiment shown in FIG. 4, a collimated input beam 14illuminates a facet area 56, and the facet area 56 transmits transmittalbeams 20 c, 20 d having a preselected angular spread 32. Those ofordinary skill in the art of this invention are familiar with angularspread. Angular spread, or angular divergence, is the increase innominal beam size over a finite propagation distance expressed as anangle in radians or degrees. In any given facet area 56, the plateaus 50and vias 52 diffract the input beam 14 such that the transmittedportions 20 c, 20 d of the input beam 14 have a preselected angularspread. The angular spread provided by a facet area 56 is selected bythe designer of the hologram 54 such that a desired output beam istransmitted to the output plane. Preferably, the designer can preselectan angular spread of from plus/minus zero to plus/minus ninety degrees.In FIG. 6, a facet area 56 (shown in side view) illuminated by a portionof the input beam 14 is shown providing an angular spread 32 ofplus/minus twenty degrees for beams 20 e, 20 f transmitted to the outputplane 18.

The choice of angular spread depends upon the application for which thebeam homogenizer is used and the desired output beam. The angular spreadis selected by the designer as needed for the application at hand. Forexample, an illumination system for machine vision may require uniformillumination across a 10 degree×15 degree rectangular area. The designerwould chose angular spread for the beam homogenizer to obtain such adesired illumination.

With a facet area 56 that transmits a beam with a predesigned angularspread, the designer can control the angle over which the transmittedlight 20 c, 20 d, 20 e, 20 f is spread. Preferably, the facet areas 56of the hologram 54 provide angular spread such that the target area 16illuminated 34 on the output plane 18 is larger than the illuminatedfacet areas 56.

In the embodiment shown in FIG. 4, the hologram 54 is designed such thata preselected target area 16 of the output plane 18 is illuminated. Theembodiment shown in FIG. 4 homogenizes spatial power over thatpreselected target area. That is, the embodiment provides a constant,preselected power distribution at the output plane even if the powerdistribution of the incident beam upon the surface of the hologramdeviates from design parameters.

The designer can choose any particular shape for the target area 16. Thedesigner designs the plateaus 50 and vias 52 such that a target pattern16 of a desired shape may be projected upon the output plane 18 from thehologram 54. The spatial frequency content of the hologram is designedto produce a desired pattern. That is, the size and orientation of theplateaus 50 and vias 52 are designed to produce a desired pattern.Preferably, the design takes place using a computer.

In the embodiment, the facet areas 56 transmit portions 20 c, 20 d ofthe input beam 14 at predetermined, designed angular spreads. Bytransmitting light at various, predetermined angular spreads, the facetareas 56 of the hologram 54 projects spatial power in predeterminedtarget patterns 16 onto the output plane 18. Essentially, a targetpattern is made up of illuminated areas 34 of the output plane 18adjacent to non-illuminated areas 36 of the output plane 18. Thehologram 54 is designed such that the spatial energy is transmitted topredetermined target areas 34 of the output plane and relatively nospatial energy is transmitted to other predetermined areas 36 of theoutput plane 18, thus projecting a predetermined target pattern 16 ontothe output plane 18.

The target pattern shown in FIG. 4 is a circular pattern. This patternis shown in a front view in FIG. 8. In FIG. 8, a circular target pattern16, that is, a preselected power distribution pattern in which thespatial power is distributed at the output plane 18 in a circle withrelatively no distribution around that circle, is shown. That is, FIG. 8shows a preselected power distribution pattern in which spatial power isdistributed at the output plane 18 such that an illuminated circulararea 34 is surrounded by a relatively non-illuminated area 36 at theoutput plane 18. The pattern shown in FIG. 8 was produced by thehologram shown in FIG. 7.

Other patterns, such as the one shown in FIG. 9, can be selected by thedesigner of the homogenizer 10 for projection. FIG. 9 shows adoughnut-shaped target pattern 16 at the output plane 18. That is, FIG.9 shows a preselected power distribution pattern in which the spatialpower is distributed at the output plane 18 such that a relativelynon-illuminated circular area 36 is surrounded by a ring of illuminatedarea 34, which is in turn adjacent to a relatively non-illuminated area36. Non-circular patterns, such as the shape of a flower or rectangle,may also be chosen.

The homogenizer 10 projects patterns having uniform spatial power onilluminated areas 34 of the output plane 18. The patterns 16 shown inFIG. 8 and FIG. 9 have relatively uniform spatial power over theirilluminated areas 34.

The embodiment shown in FIG. 4 is highly useful for photolithography.Photolithography is essentially the process of exposing patterns in aphotoreactive media. This process is used to fabricate integratedcircuits. The patterns to create these sophisticated devices must beimaged with high fidelity and maximum resolution in the photolithographyprocess.

Photolithographic exposure systems are used to image the pattern of amask onto a wafer for the purposes of exposing resist on the wafer in apredetermined pattern. Subsequent processing of the wafer results in thecompletion of layers that eventually form the desired device, such as anintegrated circuit.

When the mask is used in a projection lithography system, such as alaser stepper with a 5:1 or 10:1 reduction ratio, the mask is oftenreferred to as a reticle. The reticle or mask is typically formed bychrome regions on a transparent substrate. The chrome regions of themask block the incident light, thereby imposing the pattern of the maskas an intensity variation on the light. In a 5× laser stepper, thepattern of the reticle is reduced by a factor of 5 as imaged onto awafer. Typically, in this application, the beam illuminating thediffractive is relatively uniform and has a rather narrow cone angle ofdivergence, i.e., limited spatial and angular energy distributions.

The present invention allows for the control of the angular distributionof the light on the wafer. By modifying the particular angulardistribution of the light illuminating the wafer, one can extend thedepth of the field and resolving power of photolithographic exposuresystems. Additionally, the present invention is advantageous inphotolithography because it is relatively insensitive to fluctuations inpositioning of the incident beam and to fluctuations in the spatialenergy distributions of the incident beam.

By using the present invention, the intensity, angular frequencycontent, and pupil pattern shape of the exposure light used inphotolithography can be controlled. Such control can improve theresolution of the image of the master pattern on the integrated circuitwafer. It has been found that for some master patterns, it is better forthe light to illuminate at certain angles or ranges of angles. Thehomogenizer provides light at that angle while homogenizing the spatialenergy provided, as well as providing control of other attributes of theexposure light. Such control can improve yields in semiconductor chipmanufacturing and other areas in which photolithograhpy is used. Forexample, referring to FIG. 10, a light source (not shown) illuminates ahomogenizer 10 (shown in side view) of the embodiment of FIG. 4 with acollimated input beam 14. The homogenizer 10 transmits a majorityportion of the incident light to the mask 60 at a desired angularspread, and with a preselected intensity and spatial power distribution,wherein the preselected spatial power distribution is uniform. Thistransmitted portion 20 f, 20 g acts as the exposure light. A master, ormask, 60 is placed close to the homogenizer 10 such that there is noappreciable change in power distribution between the homogenizer 10 andthe mask 60, and thus the beam is uniform at both the homogenizer 10 andthe mask 60. Preferably, the mask 60 is not placed at the output plane18 (not shown). The master 60 is illuminated with the desired exposurelight 20 f, 20 g. The portion of the exposure light 20 f, 20 g that isnot blocked 20 h by the master 60 is transmitted by the master 60 andilluminates a lense 62. This portion 20 h has the desired, preselectedangular spread. The lense 62 in turn transmits the incident light 20 hsuch that the master 60 is imaged onto the subject wafer 64 with, forexample, photoresist (not shown). The lense 62 provides a desiredreduction factor. This embodiment images the master 60 onto the subjectwafer 64 in a desired manner, particularly a desired angular spread.Thusly, the wafer is exposed in the preselected, desired manner with anexposure light with desired and optimized attributes, and a copy isprovided. In this manner, by optimizing the exposure light as desiredand needed, higher yields during, for example, semi-conductor chipmanufacturing can be had. The homogenizer can be used to, for example,block zero to two degrees, allow two to four degrees, and block fromfour degrees onward. Or, for example, a top-hat, from plus three degreesto minus three degrees can be provided by the homogenizer. Control ofthe angular spread and the frequency content of the transmitted light isdone by design of the plateaus 50 and vias 52 of the homogenizer 10,preferably using a computer.

Preferably, the hologram 54 is a Computer Generated Hologram. Alsopreferably, the attributes of the Computer Generated Hologram, includingangular spread, are designed and chosen using a computer. The attributesare chosen to generate a desired, predetermined illuminated target area,a desired, predetermined spatial power distribution on the output plane,and other desired attributes as needed.

In designing the invention, the designer determines the angulardistribution desired. The designer designs a Fourier transform hologram,with the intensity distribution in the Fourier plane corresponding tothe desired angular range. For example, the designer may choose a ringgoing from four degrees to seven degrees, and thus three degrees wide.This target would result in a hologram with a far-field diffractionpattern in output plane 18 of a ring. Alternatively, when used in thephotolithography system of FIG. 10, the hologram's output intensity inplane 60 would be uniform, but its angular distribution in plane 60would be between four and seven degrees. The Fourier Transform hologramhas a diffractive fringe pattern, or a phase-transmittance pattern, suchthat the Fourier Transform of that pattern corresponds to a desiredtransmission over a desired angular region. How to design a Fouriertransform hologram is known to those of ordinary skill in the art ofthis invention. Preferably, the Fourier Transform is completed on acomputer.

Typically, when taking the Fast Fourier Transform, the pattern iscalculated only at discrete points. Typically, the Fourier Transformhologram is replicated in order to avoid or reduce speckle. Speckle arevery bright and very dark spots of light that occur due to interferencein coherent systems. In the present invention, the pattern is notreplicated. The preferred method to design the Fourier TransformHologram of this invention is to use an iterative computer optimizationtechnique, such as the IDO method mentioned above and described inIterative Encoding of High-Efficiency Holograms for Generation of SpotArrays, Optics Letters, Vol. 14, pp. 479-81, 1989 by co-inventor Feldmanet al. (the disclosure of which is hereby incorporated by reference), inwhich the output is the Fourier Transform plane and the input is the CGHplane. In the preferred embodiment, incoherent or partially incoherentlight, in addition to designing the hologram with a very large number ofpixels, will avoid the occurrence of speckle. A hologram with a verylarge number of pixels can provide an image at the output plane that isnearly continuous. A Fourier Transform hologram has the property thateach point in the output plane receives a contribution from every facetarea in the hologram.

Once designed and manufactured as described above, the hologram isplaced in an illumination system. When illuminated with a collimatedbeam of arbitrary intensity distribution, the hologram of this exampletransmits a ring corresponding to the desired angle in a planerelatively far from the CGH. If the beam is not collimated, then thering will be relatively wider, with a width dependent on the precisecone angle of divergence of the incident beam. In a plane relativelyvery close to the CGH, there will be seen a beam with the same intensityas the beam that illuminated the CGH, but in the relatively very closeplane, the beam will contain an angular spread corresponding to that ofthe CGH combined with that of the incident beam.

In the embodiment shown in FIG. 4, the hologram 54 is designed such thatonly designated target areas of the output plane are illuminated. Thatis, the hologram is designed such that a preselected spatial powerdistribution is incident upon the output plane.

Referring to FIG. 4, a collimated input beam 14 is incident upon ahomogenizer 10 having a hologram 54, said hologram having an array 11 ofsixteen facet areas 56 designated by dashed lines (only one of thesixteen facet areas, or facets, is shown in FIG. 4 with dashed lines,all sixteen referred to are shown in dashed lines in FIG. 7). Each ofthese facet areas 56 has a irregular pattern of plateaus 50 and vias 52.No two of these facet areas 56 are alike. Each of these facet areas 56is constructed to direct any portion of a collimated optical beam 14that is incident thereon onto a target 16 at an output plane 18. Aportion of the collimated beam 14 is transmitted through each of thefacet areas 56. This portion 20 c, 20 d has a preselected angular spreadprovided by the facet area 56. As described in the description ofprevious embodiments, this portion, shown representatively as 20 c, 20d, overlap the target 16. By this overlapping, spatial energy variationthat was present on the incident beam 14 is not present at the outputplane 18. Thus, at the output plane, there is uniform spatial powerdistribution throughout the illuminated portion 16, 34 of the plane 18.This distribution or pattern forms a beam 22. The target area, theilluminated portion, is a preselected shape. The illuminated portion 16,34 of the plane is preselected. In FIG. 4 it is a preselected circle.The illuminated portion 16, 34 of the plane does not have undesiredintensity variations from sharp edges or breaks between facets becausesuch sharp edges or breaks have been removed by the use of irregularlypatterned facets with irregularly patterned plateaus and vias.

In the present invention, the designer can select uniform magnitudes ofspatial power for the illuminated areas of the output plane. Also, inthe present invention, the designer can select different magnitudes ofspatial power for different illuminated areas of the output plane. Forexample, in FIG. 8, the designer could select a spatial power level ofmagnitude 1 (arbitrary units) for the upper half of the illuminated 34circle 16, and a spatial power level of magnitude 1.5 (arbitrary units)for the lower half of the illuminated 34 circle 16. Thus, the spatialpower distribution includes both the shape of the illumination patternon the output plane and the spatial power distribution within theilluminated portion. The spatial power distribution selected, though, ishomogenized and is therefore the power distribution remains unaltereddespite changes in the input beam power distribution. The spatial powerdistribution selected by the designer will depend upon the applicationat hand.

A phase-transmittance pattern is a mathematical description of thediffractive fringe pattern. That is, the physical diffractive fringepattern seen, for example, in FIG. 7 can be described mathematically,and a phase-transmittance pattern describes it mathematically. Personsof ordinary skill in the art would understand phase-transmittancepatterns and their mathematical relation to fringes and diffractivefringe patterns.

Those of ordinary skill in the art of this invention will know how totake the Fourier Transform of a phase-transmittance pattern. The FourierTransform hologram has a diffractive fringe pattern, or aphase-transmittance pattern, such that the Fourier Transform of thatpattern corresponds to a desired transmission over a desired angularregion. The Fourier Transform is preferably completed using a computer.

Binary phase elements are elements used in construction or manufactureof optical elements such as holograms. Persons of ordinary skill in theart of the present invention will be familiar with binary phaseelements. U.S. Pat. No. 4,895,790 discloses the construction of suchbinary phase elements and the disclosure of that patent is incorporatedherein.

Changes in construction will occur to those skilled in the art andvarious apparently different modifications and embodiments may be madewithout departing from the invention. The material set forth in theforegoing description and accompanying drawing is offered by way ofillustration only. It is therefor intended that the forgoing descriptionbe regarded as illustrative rather than limiting and that the inventionbe only limited by the scope of the claims.

We claim:
 1. A diffusing element for use as a reticle in a lithographicsystem, the diffusing element comprising: a substrate having alithographically created structure on its surface, which structureconverts an input beam illuminating at least part of said structure intoan output beam having a preselected energy distribution in an outputplane of the lithographic system, wherein substantially all of saidoutput beam is non-orthogonally incident on the output plane, the energydistribution of the output beam being substantially independent ofposition and spatial energy distribution of the input beam, bothdiffusing and shaping of the output beam being provided by saidstructure.
 2. The diffusing element of claim 1, wherein said structuresubstantially eliminates the output beam incident on a center of theoutput plane.
 3. The diffusing element of claim 1, wherein saidstructure forms a ring in the output plane.
 4. The diffusing element ofclaim 1, wherein the energy distribution over the angular distributionof the output beam is substantially uniform.
 5. The diffusing element ofclaim 1, wherein the diffusing and shaping are provided by a singlesurface.